From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-12.1 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E2C7FC433DF for ; Thu, 25 Jun 2020 02:26:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BF5D720849 for ; Thu, 25 Jun 2020 02:26:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="AZcqU08v" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389283AbgFYC0v (ORCPT ); Wed, 24 Jun 2020 22:26:51 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:48668 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2388930AbgFYC0u (ORCPT ); Wed, 24 Jun 2020 22:26:50 -0400 Received: from mail-ot1-x344.google.com (mail-ot1-x344.google.com [IPv6:2607:f8b0:4864:20::344]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4FE58C061573; Wed, 24 Jun 2020 19:26:50 -0700 (PDT) Received: by mail-ot1-x344.google.com with SMTP id 5so1883931oty.11; Wed, 24 Jun 2020 19:26:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=ZQNZYHZs3Rf5u37OLrL6y0pvgTgU4h8M5jVmsv5Nrec=; b=AZcqU08vwZw9lrCsZsKBlLyXkNyZa4Z91/EmPbGeOoW2ao/UQMOSfq81bN29+UrCso 7XPCJ68zo766UgdGF3xas4X6/4NGojSucMUgoIYWA6qPtQ80X69hWEj5o2B37QuhSRyy p3T5MPqVetLqV6OVPTWY3BhiYkewqHCVhjDURFb/o2OBxrl4wtZ70UcwJ6PC2x35PBHU a+5QytEgeiUH6+TzQCmECEq9Gjrx76ilvOkvVCK2qPGIbt8zlhiBAAvrmfbAx2O7mbJe QlK6xMcNBCWbtNdDmUQ+DHQHM9dT2zR7aGR7uZ05zx+3QEHSIhDQ9TVdZSL7xdC/foKW khWA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=ZQNZYHZs3Rf5u37OLrL6y0pvgTgU4h8M5jVmsv5Nrec=; b=oWjQg2G3t7YzrkI6b9s2jthNsFbPEEdm/EV7rpGHxglhHuxVz9mQObmV1oXDq09hHd bdq6NkVBHa8jIgBcf2lAp27AWahE40tFTthn3nQyALdQUacqi/JkTHFM3ITRIqMmQSkc fvhVHnw9VFXPkQTn/VgYqOxPBqvWFPBcXB/uq68NsrnDw4O/zt3hlil69OfL8NXhScEo 2yb0XPqZsr8/28kKaGWZPG7BTnScWntZcJ+DVc5B3o1p5a5TCa0korLN5jQDNthI0D2b TDuyyZq0xYXyb7EEEiE9g2PUPtXnrMqXflPh2Vazp0AuROMzAyLFVGW6jo9EKkuGxTe8 hQTg== X-Gm-Message-State: AOAM531CzRjCaM9dOx1FmuvK4Aq75ozZIBjo3LISkKIw+vNmtRqSWmzY h0NoKsULOYe32VJdt4+mJFjbOVAJGcU= X-Google-Smtp-Source: ABdhPJw1AzgRgryeo1cu/6FpvQVFO+54yG3gpbWyWO2n0QHC6FG7I1bntUQA1AKQNkHP8ISfYtWv5g== X-Received: by 2002:a9d:32a1:: with SMTP id u30mr5549372otb.264.1593052009657; Wed, 24 Jun 2020 19:26:49 -0700 (PDT) Received: from ubuntu-n2-xlarge-x86 ([2604:1380:4111:8b00::3]) by smtp.gmail.com with ESMTPSA id p1sm5444990oot.34.2020.06.24.19.26.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 24 Jun 2020 19:26:49 -0700 (PDT) Date: Wed, 24 Jun 2020 19:26:47 -0700 From: Nathan Chancellor To: Sami Tolvanen Cc: Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , "Paul E. McKenney" , Kees Cook , Nick Desaulniers , clang-built-linux@googlegroups.com, kernel-hardening@lists.openwall.com, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH 02/22] kbuild: add support for Clang LTO Message-ID: <20200625022647.GB2871607@ubuntu-n2-xlarge-x86> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624203200.78870-3-samitolvanen@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200624203200.78870-3-samitolvanen@google.com> Sender: linux-pci-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org Hi Sami, On Wed, Jun 24, 2020 at 01:31:40PM -0700, 'Sami Tolvanen' via Clang Built Linux wrote: > This change adds build system support for Clang's Link Time > Optimization (LTO). With -flto, instead of ELF object files, Clang > produces LLVM bitcode, which is compiled into native code at link > time, allowing the final binary to be optimized globally. For more > details, see: > > https://llvm.org/docs/LinkTimeOptimization.html > > The Kconfig option CONFIG_LTO_CLANG is implemented as a choice, > which defaults to LTO being disabled. To use LTO, the architecture > must select ARCH_SUPPORTS_LTO_CLANG and support: > > - compiling with Clang, > - compiling inline assembly with Clang's integrated assembler, > - and linking with LLD. > > While using full LTO results in the best runtime performance, the > compilation is not scalable in time or memory. CONFIG_THINLTO > enables ThinLTO, which allows parallel optimization and faster > incremental builds. ThinLTO is used by default if the architecture > also selects ARCH_SUPPORTS_THINLTO: > > https://clang.llvm.org/docs/ThinLTO.html > > To enable LTO, LLVM tools must be used to handle bitcode files. The > easiest way is to pass the LLVM=1 option to make: > > $ make LLVM=1 defconfig > $ scripts/config -e LTO_CLANG > $ make LLVM=1 > > Alternatively, at least the following LLVM tools must be used: > > CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm > > To prepare for LTO support with other compilers, common parts are > gated behind the CONFIG_LTO option, and LTO can be disabled for > specific files by filtering out CC_FLAGS_LTO. > > Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in > follow-up patches. > > Signed-off-by: Sami Tolvanen > --- > Makefile | 16 ++++++++ > arch/Kconfig | 66 +++++++++++++++++++++++++++++++ > include/asm-generic/vmlinux.lds.h | 11 ++++-- > scripts/Makefile.build | 9 ++++- > scripts/Makefile.modfinal | 9 ++++- > scripts/Makefile.modpost | 24 ++++++++++- > scripts/link-vmlinux.sh | 32 +++++++++++---- > 7 files changed, 151 insertions(+), 16 deletions(-) > > diff --git a/Makefile b/Makefile > index ac2c61c37a73..0c7fe6fb2143 100644 > --- a/Makefile > +++ b/Makefile > @@ -886,6 +886,22 @@ KBUILD_CFLAGS += $(CC_FLAGS_SCS) > export CC_FLAGS_SCS > endif > > +ifdef CONFIG_LTO_CLANG > +ifdef CONFIG_THINLTO > +CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit) > +KBUILD_LDFLAGS += --thinlto-cache-dir=.thinlto-cache > +else > +CC_FLAGS_LTO_CLANG := -flto > +endif > +CC_FLAGS_LTO_CLANG += -fvisibility=default > +endif > + > +ifdef CONFIG_LTO > +CC_FLAGS_LTO := $(CC_FLAGS_LTO_CLANG) > +KBUILD_CFLAGS += $(CC_FLAGS_LTO) > +export CC_FLAGS_LTO > +endif > + > # arch Makefile may override CC so keep this after arch Makefile is included > NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include) > > diff --git a/arch/Kconfig b/arch/Kconfig > index 8cc35dc556c7..e00b122293f8 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -552,6 +552,72 @@ config SHADOW_CALL_STACK > reading and writing arbitrary memory may be able to locate them > and hijack control flow by modifying the stacks. > > +config LTO > + bool > + > +config ARCH_SUPPORTS_LTO_CLANG > + bool > + help > + An architecture should select this option if it supports: > + - compiling with Clang, > + - compiling inline assembly with Clang's integrated assembler, > + - and linking with LLD. > + > +config ARCH_SUPPORTS_THINLTO > + bool > + help > + An architecture should select this option if it supports Clang's > + ThinLTO. > + > +config THINLTO > + bool "Clang ThinLTO" > + depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO > + default y > + help > + This option enables Clang's ThinLTO, which allows for parallel > + optimization and faster incremental compiles. More information > + can be found from Clang's documentation: > + > + https://clang.llvm.org/docs/ThinLTO.html > + > +choice > + prompt "Link Time Optimization (LTO)" > + default LTO_NONE > + help > + This option enables Link Time Optimization (LTO), which allows the > + compiler to optimize binaries globally. > + > + If unsure, select LTO_NONE. > + > +config LTO_NONE > + bool "None" > + > +config LTO_CLANG > + bool "Clang's Link Time Optimization (EXPERIMENTAL)" > + depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD I am curious, what is the reason for gating this at clang 11.0.0? Presumably this? https://github.com/ClangBuiltLinux/linux/issues/510 It might be nice to notate this so that we do not have to wonder :) Cheers, Nathan